Absent Coefficient in Linear Form

as Maximoff wrote:
> New question #95954 on DOLFIN:
> https://answers.launchpad.net/dolfin/+question/95954
>
> Hello
>
> I have a ufl file like so (for the Heat Equation with time-stepping via the theta-method):
>
> element = FiniteElement("Lagrange", "triangle", 2)
>
> v = TestFunction(element)
> u = TrialFunction(element)
> kappa = Coefficient(element)
> u_old = Coefficient(element)
> f = Coefficient(element)
>
> dt = ....
>
> a = u*v*dx + theta*dt*kappa*inner(grad(u), grad(v))*dx
> L = ((u_old + dt*f)*v - (1-theta)*dt*kappa*inner(grad(u), grad(v)))*dx
>
> The problem is that I can't assign kappa to L.kappa in the linear form, i.e.,
>
> L.kappa = kappa
>
> This is fine though for the Bilinear form.
>
> On examining the generated header file, it turns out that the LinearForm class has no member kappa which is exactly the error message given.
>
> What am I doing wrong?
>

Are you setting theta=0? In this case, UFL will discard the entire
product involving theta.

Also, your form is not correct. You cannot have the trial function u
appearing in L.

Garth

> Best wishes
>
> Pietro
>
>

I intend to use different theta values but in this particular instance, theta = 1.

You're right, L should read:

L = ((u_old + dt*f)*v - (1-theta)*dt*kappa*inner(grad(u_old), grad(v)))*dx

Even then, kappa is absent from the LinearForm class and hence, can't be set by

L.kappa = kappa

I wonder why this is the case.

Pietro Maximoff wrote:
> Question #95954 on DOLFIN changed:
> https://answers.launchpad.net/dolfin/+question/95954
>
> Status: Answered => Open
>
> Pietro Maximoff is still having a problem:
> I intend to use different theta values but in this particular instance,
> theta = 1.
>
> You're right, L should read:
>
> L = ((u_old + dt*f)*v - (1-theta)*dt*kappa*inner(grad(u_old),
> grad(v)))*dx
>
> Even then, kappa is absent from the LinearForm class and hence, can't be
> set by
>
> L.kappa = kappa
>
> I wonder why this is the case.
>

(1-theta) = 0 if theta=1 -> (1-theta)*dt*kappa*inner(grad(u_old) = 0 ->
UFL throws it away.

Garth

Ahh... I see. Thanks Garth. I just assumed that even though that part of the expression will be zero, kappa'll still be generated.